Audio transmission circuit



Patented Feb. I, 1944 2,340,364 VAUDIO TRANSMISSION CIRCUIT Alda V. Bedl'ord, Collin io Corporation of Delaware gswood, N. J., assignoi to America, a, corporation of Application August 22, 1942, Serial No. 455,734

4 Claims.

utilization circuit. 7 In radio broadcasting it is desirable to broadmission is not especially limited at quencies, but is limited uniformly transmitter to However, when this is done the peaks of the audio signal are Another object of this invention is to provide a novel method of transmitting audio signals used with advantage to increase the signal-to-noise ratio for film or disc recording,

telephone communication or frequency modulation,

A more specific generated by the saturating step.

'I 'he novel features which I belive to be charac- In the drawing,

Fig. 1 schematically shows a system embodying the invention;

Fig. 2 illustrates the overlapping relation of the several audio band-pass filters used before and after the limiting amplifiers;

Fig. 3 shows a type of limiting amplifier that may be employed;

Fig. 4 graphically represents the effect of the as from 162 to 290 initial frequencies within a specific band such cycles.

In this example, the lowest harmonic is double 162 which is 324, and the highest diflerence frequency obtainable is 128 cycles. These are outside the band. or course the loud passages are made less loud by my method, but this is acceptable. The objectionable spurious frequencies usually associated with saturation are not heard. It is, also, pointer. out that in the several bands given as examples herein, each represents slight- 1y less than an octave. This is to allow for some overlap of adjacent bands, since the filters are limited in the sharpness of cut-off. If the saturating amplifiers are of the push-pull type, or by other means are made symmetrical in their operation, each band may be made to include a frequency range of slightly less than 3:1, since only the second or higher harmonics are generated.

Referring, now, to Fig. 1 there is shown a system which embodies the invention. The various networks are schematically represented, since those skilled in the art are fully acquainted with the specific nature of the individual circuits which may be used. The source of audio waves i may be a microphone, a reproducer of a receiver, the electric pick-up of a record reproducer or a photocell pick-up device. In general, the numeral l'may indicate some device which converts sound waves into audio frequency waves By way of specific illustration, let it be assumed that source I is the microphone of a broadcast transmitter.

The audio waves are amplified by amplifier 2. They are then applied to a plurality of band pass filters F1 to F9, inclusive. The specific paths to filters F5 to Ft are omitted, but are suggested by dotted arrows. The filters F1 to F9 have respective pass band characteristics as indicated in Fig. 2. From this figure it will be noted that filter F1 will pass audio signal energy of the narrow band of 50 to 90 cycles; F2 passes 90-162 cycles; Fa passes 162-290 cycles; F4 passes 290-525 cycles; F5 passes"525-945 cycles; Fe passes 945-1700 cycles; F1 passes "1700-3060 cycles; Fa passes 3060-5500 cycles and F9 passes 5500-9900 cycles. The band filters F1 to F9 are well known in the art. Each band represents slightly less octave, or a 2:1 range of frequency. This is to allow for some overlap of adjacent bands, since the filters are limited in the sharpness of cut-off. The response of each two adjacent filters in the overlap region is such that their sum is substantially 100% in order to provide a uniform overall response.

The output of each of filters F1 to F9 is passed through a respective limiting amplifier LA1 to LAa, respectively. These limiting amplifiers may be ofany well known form. The signal energy in each narrow band is separately amplified by a saturating, or limiting, amplifier so as to limit any excessive peaks. In Fig. 3 there is shown a well-known type of limiting amplifier which maybe employed. This type of amplifier limits both the positive and negative swings, as illustrated at SB in Fig. 4. 4

Referring specifically to Fig. 3, the input energy from a respective band is applied through condenser 5 to grid 6 of tube 1. The cathodes of tubes 1 and 8 are connected to the upper end of resistor 9. The grid 6 returns to the grounded end of cathode resistor 5 by a grid return resistor Ill. The grid ll of tube 8 returns to ground, and the plates of tubes 1 and 8 are conthan an nected to the positive terminal of an energizin source. The limited audio voltage is taken of! the plate end of plate resistor 12, and transmitted through coupling condenser II to the output circuit.

In operation. tube 1 limits the negative swing of the applied signal by the plate current going to zero, i. e., to cut-oil. The cathode output of tube 1 is then applied to tube 8 by way of its cathode, the grid being grounded. The polarity of the cathode utput of tube 1 is the same as that applied to its grid, but since this output is applied to the cathode of tube 8 instead of to its grid, the result is that tube I is in effect driven with the opposite polarity. Therefore, tube 8 limits the opposite side of the wave when it swings to plate current cut-oil. This circuit is y advantageous as a limiter in that the clipping level for both tubes is fixed with respect to the alternating current axis of the original wave applied to tube 1. If the coupling between the two stages was not D. C. in character, the clipping level in the second tube would change with the shift in the alternating currentaxis caused by the clipping of only one side of the wave by tube 1.

The output voltage of each limiting amplifier is passed through a respective band pass filter.

Thus, filters F10 to F90 may correspond in construction and characteristics to filters F1 to F9 respectively. These filters F10 to F90 remove any harmonics caused by the prior limiting action, as explained previously. The output energies of filters F10 to F90 are fed to mixer 3 where the energies are recombined or added to form a complete sound wave. After amplification at 4, the audio signal energy may be fed to the transmitter apparatus, or to a recording device, or may be transmitted over a line.

In Fig. 4 there is shown the effect of the present process on the audio wave. The figure is intended to apply to each narrow band only. The wave SA represents the original wav Limiting causes the flattening effect at Sn. The effect of Flo-F90 is to smooth the wave as at Sc. A slight amount of phase'shift (not shown in the figure) may occur, but a reasonable phase shift in the filters would cause no harm. The loud passages are made less loud by this method, but that is acceptable. The objectionable spurious frequencies usually associated with saturation are not heard.

While I have indicated and described a system for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A method of audio wave transmission which comprises dividing the audio wave energy into a plurality of narrow overlapping audio bands, amplifying each narrow band and concurrently limiting such amp cation, filtering each amplified narrow band to remove spurious frequencies caused by limiting action, and combining the filtered energies of the various bands to provide a wave having energy at all frequencies occurring in the original wave.

2. A method for limiting the amplitude of an original audio signal which consists in dividing the original audio signal into several narrow slightly less than an octave, subjecting the audio action, and combining the energies of the narenergy of each narrow band to limiting amplirow hands to form the original audio signal wave. ilcation, removing any harmonics caused in each 4. A method of transmitting a sound wave narrow band by said limiting amplification, and which includes converting the wave into audio recombining the energies of the several narrow 5 signal energy, dividing the audio energy into a bands. v plurality of overlapping narrow bands of slightly 3. A method of transmitting a sound wave less than an octave, limiting the amplitude of which includes converting the wave into audio energy in each narrow band filtering the energy limited energy of each narrow band to filtering original wave relative to the average amplitude. to remove spurious frequencies causedby limiting I ALDA V. BEDF'ORD. 

